R&S FSH
Cable Measurements
1145.5973.12 4.129
E-14
only as many points as permitted by the span that has been set. Thus, the length resolution decreases
and the span is reduced.
The following two screenshots show the measurements of the faults of a 22 m cable at a set cable
length of 25 meters, measured once with automatic setting of the span and once with the span reduced
to 400 MHz. Both measurements clearly show the cable coupling used at 12 m and the end of the cable
which has a 50
[
termination.
Measurement with automatic setting of the span
(span = 2.025 GHz)
Measurement with 400 MHz span
The trace on the right shows the fault location at the end of the cable clearly wider than the left one
which was measured with optimum span. The reason is the reduced number of points in the calculation
with reduced span. The points in reduced span are calculated using the following formula:
1024
AUTOSPAN
SPAN
N
=
,
where
N = number of measurement points
SPAN = manually set span
AUTOSPAN = span used by the R&S FSH in automatic setting of the span
In the above measurement example, the resolution is thus 202 points for a cable length of 25 m, i.e. the
distance is approx. 12.4 cm between the two measurement points.
Selecting the center frequency
The R&S FSH’s center frequency should be as close to the cable under test’s operating frequency as
possible (for example the transmission frequency of the antenna connected to the cable). Cable
attenuation increases with increasing frequency. This means that both the incident wave and the
reflected wave from the end of the cable or at any faults is attenuated more at higher frequencies. This
restricts the dynamic range at higher center frequencies. Therefore, never select a center frequency
that is higher than necessary.
In the case of short cable lengths and automatic setting of the span, the R&S FSH uses its entire
frequency range for the measurement. It automatically uses 1.505 GHz as the center frequency.
After the span is reduced, the R&S FSH can be set to the desired center frequency.
深圳市盛腾仪器仪表有限公司
Tel:0755-83589391
Fax:0755-83539691
Website: www.Sengt.com
